Fuel cell is an energy conversion device, which can directly convert chemical energy of hydrogen-rich fuel into electrical energy. Its by-products are almost non-polluting (e.g., water or methanol). Fuel cell has become one of the most noticeable and anticipated generators of clean energy with its high power generation efficiency, low noise, zero carbon footprint, no memory effect and no grid-dependence.
There are many types of fuel cell, all of whose major structural design is two layers of electrode and an electrolyte between the two electrodes. The type and phase (liquid or solid) of the electrolyte depend on the type of the fuel cell. The main function of the electrolyte is to separate the electrodes for preventing internal current conduction and helping conducting protons or hydroxide ions. In general, fuel cell is mainly applied in three fields, including portable power generation, fixed power generation and transportation power generation. Among them, fuel cell is applied in the field of portable power generation the most widely, including consumer electronics, auxiliary power systems, and military equipments.
PEMFC is a type of fuel cell used in the fields of portable power generation and transportation power generation. According to the working temperature, a PEMFC can be a low-temperature one and a high-temperature one. A low-temperature PEMFC is operated at a temperature of 0˜100° C.; and, a high-temperature one is 100˜200° C. However, the high-temperature one has a number of advantages as compared to the low-temperature one. For example, the poisoning effect of carbon monoxide to the activity of the catalyst can be reduced; the electrochemical reaction kinetics can be enhanced; recombination and purification of fuel can be simplified; extra humidification system is discarded; and thermal management can be simplified because a cooling device is no more required.
According to an industrial review report of Fuel Cell Today 2012, it was noted that, in 2011, world's total electricity generation of fuel cell had come to more than 100 MW; in 2012, even 175.8 MW. Therein, PEMFC accounted for 73.8 MW (approximately 42%). From a volume perspective, in 2012, the total global shipment of fuel cell came to 78.2 thousand pieces, where PEMFC accounted for 70.9 thousand pieces (about 91%). This is because PEMFC is portable, high-powered and modularizable to be widely used on various types of stationary backup powers, transportation equipments and portable electronic products.
An electrolyte membrane used in a high-temperature PEMFC is mainly a solid electrolyte membrane made of a polymer of polybenzimidazole doped with a strong acid. The most suitable strong acid to be doped into polybenzimidazole is phosphoric acid (PA), which is relatively thermal-stable and can form ionic bonding with polybenzimidazole. The general chemical structure of polybenzimidazole is as follows:

It is known that the amount of phosphoric acid contained in an acid-doped polybenzimidazole membrane will significantly affect its proton conductivity (Q. Li et al., Fuel Cells, 2004, 4 volumes, 147-159). To obtain an excellent power generation efficiency of the fuel cell, the electrolyte membrane must have excellent proton conductivity; and the higher the content of phosphoric acid contained, the higher proton conductivity the acid-doped polybenzimidazole membrane have. However, a document (Wainright et al., Journal of The Electrochemical Society, 1995, 142 volumes, L121-L123) reported that a polybenzimidazole membrane doped with phosphoric acid through a general method of being soaked in phosphoric acid can only obtain 5 phosphoric acid molecules/each polymer repeating unit, where this much content of phosphoric acid can not obtain a suitable proton conductivity under an anhydrous environment.
Therefore, in order to substantially increase the content of phosphoric acid introduced into the polybenzimidazole membrane for improving the proton conductivity of the membrane, there are currently two technical methods can be employed. A first method is to use a so-called sol-gel process to fabricate the acid-doped polybenzimidazole membrane. The sol-gel process uses polyphosphoric acid as a solvent to synthesize a polybenzimidazole. After completion of the synthesis reaction without precipitating the polymer, the reaction solution is directly processed through a membrane-gelling process. In the gelling process, the solvent of polyphosphoric acid absorbs moisture (gas) to be directly hydrolyzed into phosphoric acid so that the polybenzimidazole membrane can obtain phosphoric acid without processing soaking. This method can be found in US Patent US20070193885. The membrane fabricated through this method can have a phosphoric acid content at least 20 phosphoric acid molecules per polymer repeating unit and a proton conductivity more than 10−1 siemens per centimeter (S/cm). The second method is to fabricate a porous-structure polybenzimidazole membrane. Patent US20040058216, U.S. Pat. No. 6,187,231, U.S. Pat. No. 6,602,630 and US20120000852 describe methods for fabricating a porous-structure polybenzimidazole membrane. A document (D. Mecerreyes et al, Chemistry of Materials, 2004, 16 volumes, 604-607) also confirmed that the porous structure can make more phosphoric acid be doped into polybenzimidazole through a general soaking way due to its capillary action, where the phosphoric acid content contained can reach at least 10 phosphoric acid molecules per polymer repeating unit and the proton conductivity can reach more than 5×10−2 S/cm siemens per centimeter (S/cm).
The above two methods for fabricating a phosphoric-acid-doped polybenzimidazole membrane with high phosphoric acid content still have some problems to be improved. For example, the phosphoric-acid-doped polybenzimidazole membrane fabricated through the sol-gel process must accurately grasp the process of polyphosphoric acid absorbance and hydrolysis, while the molecular weight of polybenzimidazole must be very high. If they are not both matched, it is not possible to fabricate a uniform membrane.
Regarding a porous-structure polybenzimidazole membrane, a higher porosity makes a higher content of phosphoric acid introduced, where, however, it may be accompanied with weakened mechanical strength and reduced oxidative stability. On being applied to a PEMFC, the electrolyte membrane along with two gas diffusion electrodes (GDE) is made into a sandwich-structure membrane electrode assembly (MEA) through hot-pressing. Then, units of membrane electrodes are linked together in a parallel way to form a battery pack. Therefore, the electrolyte membrane must have a certain mechanical strength to avoid breakage on being handled and assembled into modules during the hot-pressing process. Because the fuel at cathode of fuel cell is oxygen, there are chances of generating peroxy radicals, which attack the electrolyte membrane and make it decomposed. Therefore, the electrolyte membrane must consider the stability of oxidative cleavage of its material. Furthermore, the electrolyte membrane not only has to have good ionic conductivity and electrical insulation, but also needs to avoid fuel crossover. However, when the porosity of the membrane reaches a certain level, individual pores will be in communication, which results in a connective porous structure. Such a structure may make the gaseous fuels at anode and cathode mutually cross to the opposite side to cause a substantial decline in the voltage of the fuel cell without achieving the desired power density generated by the fuel cell. Meanwhile, at the interface between the electrolyte membrane and the gas diffusion electrode at the cathode side in the fuel cell, an electrochemical reaction of oxygen dissociation along with proton combined for forming water may occurs and the water generated at the interface may easily drag out phosphoric acid adhered in the porous structure to cause acid leakage. As a result, the phosphoric acid content of the electrolyte membrane is reduced along with the proton conductivity decreased, and the power generation efficiency of the fuel cell is further affected.
On applying the porous-structure polybenzimidazole membrane as an electrolyte membrane in a fuel cell, the problems of mechanical strength and insufficient oxidative-cleavage stability can be effectively improved through a procedure of chemical or physical crosslinking. However, regarding the issues of fuel crossover and acid leakage causing serious adverse effects on power generation efficiency of the fuel cell, problems are still not solved.